Method and apparatus for controlling the optical power of an optical transmission signal

Abstract

The present invention generally relates to the field of optical transmission and particularly to a method and an apparatus for controlling the optical power of an optical transmission signal in a wavelength division multiplex optical transmission system (WDM system). The optical power of a control channel is controlled to keep the total optical power of the optical transmission signal constant. The present invention avoids the deleterious effects of stimulated Brillouin Scattering by spreading the power density spectrum of the optical control signal.

What is claimed is:  
     
       1. A method for controlling the optical power of an optical transmission signal in a wavelength division multiplex system, comprising: 
       adding an optical control signal having a variable optical power to compensate for variations in the optical power of the optical transmission signal; and  
       phase modulating said optical control signal, thereby to spread an associated power density spectrum of said optical control signal, said phase modulation comprising switching a current fed to a semiconductor laser diode between two levels, wherein the first level falls short of a threshold for laser mode operation of said semiconductor laser diode and the second level is adjusted in accordance with the variations in the optical power of the optical transmission signal.  
     
     
       2. An apparatus for controlling the optical power of an optical transmission signal in a wavelength division multiplex system, comprising: 
       a light source for generating an optical control signal having a variable optical power, said light source comprising a semiconductor laser diode;  
       a control element for detecting variations in the optical power of the optical transmission signal and controlling in response the optical power of the light source; and  
       a combining element for adding the optical control signal to the optical transmission signal; wherein:  
       the apparatus comprises an element for spreading an associated power density spectrum of the optical control signal, said spreading element adapted to phase modulate a current coupled to said semiconductor laser diode by switching said current between two levels;  
       the first of said two levels of said current falls below a threshold for laser mode operation of said semiconductor laser; and  
       the second of said two levels of said current is adjusted according to the variations of the optical power of the optical transmission signal detected by the control means.